“Boo! Did I scare you?”
“Boo” is what you say when you jump out and startle somebody who didn’t know you were there. You can also see it in the name of the game “peekaboo”. Peekaboo is not scary, but it does involve hiding and suddenly revealing yourself.
「ワッ！びっくりした？」
Boo は、いきなり飛び出して人をびっくりさせる時に使う言葉です。 Peekaboo（いないいないばあ！）の単語にも入っています。Peekaboo は別に怖い遊びではありませんが、隠れて急に現れるところは同じですね。
